The present invention relates to the field of instrumentation for differential thermal analysis and differential scanning calorimetry.
Differential thermal analysis (DTA) generally refers to a calorimetric technique for measuring physical properties of a substance by exposing the substance to different temperature regimes. DTA can be employed to measure parameters associated with phase transitions, glass transitions, polymerization/depolymerization, crystallization, softening, sublimation, dehydration, decomposition, oxidation, cure kinetics and so forth. A differential scanning calorimeter (DSC) measures temperatures and heat flows associated with energy-emitting or energy-absorbing (exothermic and endothermic, respectively) material transitions. DSCs are widely used in academic, government and private facilities for research purposes, as well as for quality control and production purposes.
Hereinafter, reference will be made to DSC, although it is to be understood to encompass DTA as well.
Typical DSC instrumentation includes the following basic components: a heated measurement chamber enclosing a sensor assembly upon which the material to be evaluated (the xe2x80x9csamplexe2x80x9d) may be placed; a furnace heater for heating the measurement chamber; and a cooling device. The cooling device acts as a heat sink for the furnace heater. The cooling device may find application when temperature in the measurement chamber is being increased or decreased.
Typical DSC instrumentation also includes control circuitry for controlling the furnace heater/cooling device so as to conform the temperature in the measurement chamber to the programmed temperature profile. The DSC instrumentation may also include output means, such as a printer or video screen or plotter, to present the results of the measurements. Results can be presented as plots of temperature difference versus absolute temperature or heat flow (e.g., watts per gram) versus absolute temperature.
In traditional DSC analysis, the measurement chamber holds a sample of interest and a reference material, which are to be subjected to a programmed temperature profile. The reference material is typically inert over the profile of interest or otherwise well understood. Typically, DSC analyses generally do not use an actual reference material; rather, the reference pan is left empty.
The sample and the reference material are placed on the DSC sensor assembly, which includes a sample position temperature detector and a reference material position temperature detector. These two temperature detectors are typically configured so that the temperature difference between the sample position and the reference material position can be directly measured. FIG. 1b of Reading, et al., U.S. Pat. No. 5,224,775 (the ""775 patent), provides an illustration of a basic DSC device. FIG. 1 of Stone, U.S. Pat. No. 3,456,490 (the ""490 patent), illustrates another configuration of a basic DSC device. The ""775 and ""490 patents are herein incorporated by reference in their entirety.
During operation, the furnace heater and/or cooling device are controlled to follow the programmed temperature profile. The temperature difference xcex94T (or heat flow into or out of) between the sample and the reference material is measured as a function of the measured sample temperature. The results, such as sudden excursions in the temperature difference xcex94T when the sample changes phase or undergoes a chemical reaction, are studied to better understand the properties and behavior of the sample.
There are other variations of such thermal analysis techniques, such as Pressure Differential Scanning Calorimetry (PDSC), Pressure Differential Thermal Analysis (PDTA), Differential Photocalorimetry (DPC), and Pressure Differential Photocalorimetry (PDPC). The invention described hereafter may be applied to such variations, which are all well known in the art.
As should be readily appreciated, it is a significant challenge to design DSC instrumentation that provides an acceptable combination of attributes, such as the temperature profile range, cooling and heating rates (how fast the measurement chamber can be cooled or heated), accuracy, and precision. The temperature profile can range from the lowest to highest achievable value, e.g., xe2x88x92150xc2x0 C. to +725xc2x0 C. for the calorimeter disclosed in the ""775 patent or xe2x88x92200xc2x0 C. to +725xc2x0 C. for the calorimeter disclosed in U.S. patent application Ser. No. 09/767,903 filed on Jan. 24, 2001, as a continuation-in-part of U.S. patent application Ser. Nos. 09/643,870 and 09/643,869. U.S. patent application Ser. No. 09/767,903 filed on Jan. 24, 2001, is hereby incorporated by reference in its entirety. Prior art DSC devices often entail unsatisfactory tradeoffs between such attributes.
Prior art devices have other drawbacks. For example, it is desirable to keep the temperature of the measurement chamber uniform so that both the sample and the reference material are exposed to the same thermal stimulus. Yet, prior art designs are often susceptible to temperature variations or gradients in the measurement chamber. Such temperature nonuniformities are difficult to predict/measure so as to compensate for them through signal processing. These phenomena can lead to measurement errors.
It has also been difficult to strike an acceptable balance between high cooling rates and temperature uniformity in prior art designs. For example, these designs may permit high cooling rates, but tend to do so at the expense of uniformity.
Finally, prior art designs have not readily lent themselves to a modular configuration that permits easy and rapid replacement of components to tailor the DSC instrumentation to the application. Even where prior art configurations might physically permit modular substitution of components (such as replacing a cooling device of a first type with a cooling device of a second type), the inherent design characteristics of prior art configurations may greatly limit the benefit of such modularity. For example, a DSC unit may permit a xe2x80x9ccooling finxe2x80x9d device to be coupled to the measurement chamber/furnace heater to provide a heat sink during above-ambient xe2x80x9chotxe2x80x9d measurements. The application is then changed so that substantially below-ambient xe2x80x9ccoldxe2x80x9d measurements are desired. The prior art design may physically permit substituting the cooling fin with a high-powered xe2x80x9cliquid cooled heat exchanger.xe2x80x9d However, the prior art""s design characteristics (e.g., very inefficient heat transmission paths) may not permit operation down to the desired low temperature even with the more effective cooling device.
To overcome these drawbacks or disadvantages in the prior art, and in accordance with the purpose of the invention, as embodied and broadly described, an embodiment of the present invention comprises a DSC coupling assembly for coupling a furnace block assembly (sometimes referred to as a xe2x80x9cDSC cell,xe2x80x9d having a furnace heater for heating a measurement chamber containing a DSC sensor upon which the sample and the reference materials are placed) to a variety of cooling devices. The DSC coupling assembly comprises a distributed thermal resistor attached to a cooling flange.
The distributed thermal resistor has the thermal characteristic of permitting moderate heat flow between a furnace heater and the cooling flange so as to support experiments in a variety of temperature regimes, high and low. The distributed thermal resistor has the mechanical characteristic of being adapted to withstand, without permanent deformation, the mechanical strains associated with the relative movement of the furnace assembly and cooling flange (due to expansion and contraction) during operation.
The cooling flange of the DSC coupling assembly is coupled to the thermal resistor. The cooling flange has the thermal characteristic of moderate conductivity permitting even and efficient heat flow through the thermal resistor. The cooling flange has the mechanical characteristic of having a standard shape profile permitting ready coupling to various cooling devices. In attaching a selected cooling device to the cooling flange, physical contact and thermal paths between the measurement chamber and cooling device are well defined and reproducible.
The advantages of the present DSC coupling assembly are numerous. The distributed, moderate heat flow through the thermal resistor permits the use of various cooling devices to support experiments in a variety of temperature regimes. The structure of the thermal resistor provides a resilient, long-life coupling assembly that will not permanently deform due to operational stresses. The efficient and even heat flow through the cooling flange maximizes temperature uniformity within the measurement chamber, while still achieving the desired high cooling and heating rates. It also permits use of the DSC coupling assembly in a wide variety of temperature regimes. A further advantage is that the configuration of the cooling flange permits attachment of selectable cooling devices in a manner that minimizes undesired heat flow from the measurement chamber.
Accordingly, an object of the invention is to provide a DSC coupling assembly that provides well defined and reproducible heat transfer.
Another object of the invention is to provide a DSC coupling assembly that permits a broad temperature profile range with heating and cooling rates that are more rapid than those achieved in the prior art.
Another object of the invention is to provide a DSC coupling assembly with improved temperature uniformity characteristics.
An object of a preferred embodiment of the invention is to provide a DSC coupling assembly with a generally modular configuration that permits effective operation with a multiplicity of cooling devices that are easily interchanged.
These and other objects of the present invention are described in greater detail in the following description of the invention, the appended drawings, and the attached claims.